Electric discharge gas lasers are well known and have been available since soon after lasers were invented in the 1960s. A high voltage discharge between two electrodes excites a gaseous gain medium. A resonance cavity containing the gain medium permits stimulated amplification of light which is then extracted from the cavity in the form of a laser beam. Many of these electric discharge gas lasers are operated in a pulse mode.
Excimer lasers are a particular type of electric gas discharge laser and have been known as such since the mid 1970s. A description of an excimer laser, useful for integrated circuit lithography, is described in U.S. Pat. No. 5,023,884 issued Jun. 11, 1991 entitled xe2x80x9cCompact Excimer Laser.xe2x80x9d This patent has been assigned to Applicants"" employer, and the patent is hereby incorporated herein by reference. The excimer laser described in Patent ""884 is a high repetition rate pulse laser. In FIG. 1, the principal elements of the laser 10 are shown. (FIG. 1 corresponds to FIG. 1 and FIG. 2 corresponds to FIG. 7 in Patent ""884.) The discharges 22 are between two long (about 23 inches) electrodes 18 and 20 spaced apart by about ⅝ inch. Repetition rates of prior art lasers, like the one described, are typically within the range of about 100 to 2000 pulses per second. These high repetition rate lasers are usually provided with a gas circulation system. In the above referred to laser, this is done with a long squirrel-cage type fan 46, having about 23 blades 48. The fan blade structure is slightly longer than the electrodes 18 and 20 and provides sufficient circulation so that at pulse operating rates, the discharge disturbed gas between the electrodes is cleared between pulses. The shaft 130 of fan 46 is supported by two ball bearings 132 as shown in FIG. 2A which is FIG. 9 of Patent ""884. The gas used in the laser contains fluorine which is extremely reactive. The fan rotor driving fan shaft 130 is sealed, within the same environmental system provided by housing structure members 12 and 14, by sealing member 136 as explained at column 9, line 45 of Patent ""884, and the motor stator 140 is outside sealing member 136 and thus protected from the corrosive action of the fluorine gas. However, bearing 132 is subjected to the corrosive action of the chamber gas as is the lubrication used in the bearing. Corrosion of the bearings and bearing lubrication can contaminate the gas.
These excimer lasers, when used for integrated circuit lithography, are typically operated on a fabrication line xe2x80x9caround-the-clockxe2x80x9d; therefore down time can be expensive. For this reason most of the components are organized into modules which can be replaced normally within a few minutes.
Excimer lasers used for lithography must have its output beam reduced in bandwidth to a fraction of a picometer. This xe2x80x9cline-narrowingxe2x80x9d is typically accomplished in a line narrowing module (called a xe2x80x9cline narrowing packagexe2x80x9d or xe2x80x9cLNPxe2x80x9d) which forms the back of the laser""s resonant cavity. This LNP typically is comprised of delicate optical elements including prisms, a mirror and a grating. As repetition rates increase maintaining stable performance by the LNP becomes a serious challenge.
Electric discharge gas lasers of the type described in U.S. Pat. No. 5,023,884 utilize an electric pulse power system such as that described in FIG. 3 to produce the electrical discharges, between the two electrodes. In such prior art systems, a direct current power supply 22 charges a capacitor bank called xe2x80x9cthe charging capacitorxe2x80x9d or xe2x80x9cC0xe2x80x9d 42 to a predetermined and controlled voltage called the xe2x80x9ccharging voltagexe2x80x9d for each pulse. The magnitude of this charging voltage may be in the range of about 500 to 1000 volts. After C0 has been charged to the predetermined voltage, a solid state switch 46 is closed allowing the electrical energy stored on C0 to ring very quickly through a series of magnetic compression circuits comprising capacitor banks 52, 62 and 82 and inductors 48, 54 and 64 and a voltage transformer 56 to produce high voltage electrical potential in the range of about 16,000 volts across the electrode which produces the discharge which lasts about 50 ns.
In prior art systems on the market the time between the closing of the solid state switch and the discharge is in the range of about 5 microseconds; however, the charging of C0 accurately to the pre-selected voltage has in the past required about 400 microseconds which was quick enough for pulse repetition rates of less than about 2,000 Hz. The reader should understand that accurate charging of C0 is very important since the control of the voltage level on C0 is in these systems the only practical control the laser operator has on the discharge voltage which in turn is the primary determiner of laser pulse energy.
Prior art excimer lasers used for integrated circuit lithography typically require a system for cooling the laser gas which is heated both by the electric discharges and by the energy input through circulating fan discussed above. This is typically done with a water cooled, finned heat exchanger shown at 58 in FIG. 1. A doubling or more of the repetition rate of a laser more than doubles the heat generated in the laser primarily because power required to circulate the laser gas increases as the cube of the required gas velocity.
When used as a light source for integrated circuit lithography, the laser beam parameters (i.e., pulse energy, wavelength and bandwidth) typically are controlled to within very tight specifications. This requires pulse-to-pulse feedback control of pulse energy and somewhat slower feedback control of wavelength of the line narrowed output beam. A doubling or more of the pulse rate requires these feedback control systems to perform much faster.
What is needed is a better laser design for a pulse gas discharge laser for operation at repetition rates in the range of about 4,000 pulses per second.
The present invention provides an excimer laser capable of producing a high quality pulsed laser beam at pulse rates of about 4,000 Hz at pulse energies of about 5 mJ or greater. A preferred embodiment is an ArF excimer laser specifically designed as a light source for integrated circuit lithography. An improved wavemeter with special software monitors output beam parameters and controls a very fast PZT driven tuning mirror and the pulse power charging voltage to maintain wavelength and pulse energy within desired limits. In a preferred embodiment two fan motors drive a single tangential fan which provides sufficient gas flow to clear discharge debris from the discharge region during the approximately 0.25 milliseconds between pulses.